The present invention relates to scrubbing of SO2 and other acid gases from the combustion products emitted by boilers and other industrial processes. The invention relates to the use of a circulating reactor in conjunction with dry particulate collection.
In prior art methods, used for cleaning of flue gases from boilers, incinerators and other combustors, the flue gases are contacted with a slurry of alkaline reagent in a reactor vessel, commonly known as a spray dryer, in which the slurry is dried, and acid gases react with the alkaline reagent to form solid particulate. In other prior-art methods, the flue gases are contacted with a dry alkaline reagent injected as a powder into a reactor vessel in which a separate cooling water stream is injected. In both prior art methods, the flue gas is humidified and cooled in the reactor, and the particulate reaction products are collected in a fabric filter or electrostatic precipitator. In both prior art methods, the alkaline reagent may consist of a portion of the collected particulate reaction products and an amount of fresh alkaline material, in most cases Ca(OH)2. In these prior art methods, the reactors were subject to internal deposition of alkaline material, and nozzles were subject to plugging, abrasive wear, and external deposits, all of which reduce the reliability of the prior art methods.
Accordingly, one object of the present invention is to overcome the problems in the prior art as noted above. Other objects of the present invention include increased efficiency of utilization of the alkaline reagent, shortened drying time, lower pressure drop across the system, higher reliability, improved turndown capability, and the ability to dispose of liquid waste steams containing dissolved alkali salts.
The present invention achieves the foregoing objectives by providing a process for removing acid gases, including SO2, from waste gases, wherein a free-flowing, moisture-containing particulate alkaline absorbent is introduced into an absorption zone through which the waste gas is passed and absorbent is entrained in the waste gas and transported to a gas/solids separator. Within the absorption zone, SO2 and other acid gases react with the particulate alkaline absorbent to form the acid salts (reaction products) of the alkaline absorbent. The waste gases and particulate solids, including reaction products flow to the gas/solids separator, wherein the particulate is separated from the gases and is recovered in the hoppers of the separator. A major portion of the recovered particulate is returned to a mixer wherein the particulate is admixed with a water suspension of calcium hydroxide or other alkaline reagent to form the free-flowing particulate alkaline absorbent, which is reintroduced in the absorption zone to remove acid gases.
For the purpose of promoting the reactions by which the acid gases are absorbed, and to avoid agglomeration of solids in the absorption zone, the moisture content of the particulate alkaline absorbent is controlled to be within 5-40 weight % based on the total weight of the absorbent feed to the absorption zone. More preferably, the moisture content of the absorbent is controlled to be within 5-20 weight %. Within the mixer, the recovered particulate is coated with a water suspension of alkaline reagent. The water suspension of the alkaline reagent is prepared externally to the mixer. The alkaline reagent is preferably calcium hydroxide produced by the reaction of water with calcium oxide.
The presence of small amounts of chloride in the particulate alkaline absorbent will improve the absorption of SO2 and reduce consumption of calcium hydroxide. The chloride may originate in the waste gases entering the absorption zone, or in one preferred embodiment of the invention, may be added to the particulate alkaline absorbent in the mixer. In this case, an aqueous solution of calcium chloride is introduced in conjunction with the calcium hydroxide suspension.
In the preferred embodiments, the process of the present invention uses flash drying.
One major advantage Of the present invention is the ability to treat aqueous waste streams originating outside the process, converting the dissolved metal alkaline salts contained therein to a dry particulate solid that is collected in the gas/solids separator. In such cases, the waste stream is introduced in the mixer in conjunction with the calcium hydroxide suspension.
Another important advantage of the present invention is the avoidance of spray nozzle plugging and abrasion associated with prior art processes wherein slurry of calcium hydroxide is contacted with the waste gas stream. By eliminating the slurry spray from the absorption zone and providing a large surface area for drying and absorption, the drying and reaction times are reduced to less than 5 seconds, and nozzle plugging and abrasive wear are eliminated. The large surface area and short drying time also help to avoid agglomeration of solids within the absorption zone.
In a preferred arrangement of the process, a supplemental water injection point is provided in the inlet to the absorption zone. This supplemental water is independent of the particulate alkaline absorbent feed. By separating a portion of the cooling water from the particulate alkaline absorbent, this preferred embodiment offers the following advantages:
1. The waste gases can be cooled during startup and during transfer of operation between plural reactors. The waste gases may be cooled when acid gas scrubbing is not required, in order to protect the gas/solids separator from excessive temperatures. The separate water injection facilitates system operation and allows for operation without add gas scrubbing.
2. By introducing a portion of the required cooling water at the absorption zone, prior to introduction of the absorbent, the amount of recycled particulate material is reduced. The use of the independent water injection may reduce the recycle material amount by up to 50%.
3. The separate water injection may also serve as a location for introducing other materials that will enhance the acid gas absorption.
The recycle silo is provided with a metering device (commonly known as a feeder) in the silo hopper, to control the flow of recycle particulate to the mixer. Excess particulate is separately discharged to a disposal system.